Solid state light sources may be utilized to provide colored (e.g., non-white) or white LED light (e.g., perceived as being white or near-white). White solid state emitters have been investigated as potential replacements for white incandescent lamps due to reasons including substantially increased efficiency and longevity. Longevity of solid state emitters is of particular benefit in environments where access is difficult and/or where change-out costs are extremely high.
A solid state lighting device may include, for example, at least one organic or inorganic light emitting diode (“LED”) or a laser. A solid state lighting device produces light (ultraviolet, visible, or infrared) by exciting electrons across the band gap between a conduction band and a valence band of a semiconductor active (light-emitting) layer, with the electron transition generating light at a wavelength that depends on the band gap. Thus, the color (wavelength) of the light emitted by a solid state emitter depends on the materials of the active layers thereof. Solid state light sources provide potential for very high efficiency relative to conventional incandescent or fluorescent sources, but solid state light sources present significant challenges in simultaneously achieving good efficacy, good color reproduction, and color stability (e.g., with respect to variations in operating temperature).
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). In the calculation of the CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference illuminant and the test source. After accounting for chromatic adaptation with a Von Kries correction, the difference in color appearance ΔEi, for each sample, between the test and reference illumination, is computed in CIE 1964 W*U*V* uniform color space. It therefore provide a relative measure of the shift in surface color and brightness of an object when lit by a particular lamp. The general color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. The CRI Ra equals 100 (a perfect score) if the color coordinates and relative brightness of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80).
Aspects relating to the present inventive subject matter may be better understood with reference to the 1931 CIE (Commission International de l'Eclairage) Chromaticity Diagram and/or the 1976 CIE Chromaticity Diagram, both of which are well-known and readily available to those of ordinary skill in the art. The CIE Chromaticity Diagrams map out the human color perception in terms of two CIE parameters x and y (in the case of the 1931 diagram) or u′ and v′ (in the case of the 1976 diagram). The 1931 CIE Chromaticity Diagram is reproduced at FIG. 1, and the 1976 CIE Chromaticity Diagram (also known as (u′v′) chromaticity diagram) is reproduced at FIG. 2. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The 1976 CIE Chromaticity Diagram is similar to the 1931 Diagram, except that the 1976 Diagram has been modified such that similar distances on the Diagram represent similar perceived differences in color.
Since similar distances on the 1976 Diagram represent similar perceived differences in color, deviation from a point on the 1976 Diagram can be expressed in terms of the coordinates, u′ and v′, e.g., distance from the point=(Δu′2+Δv′2)′1/2, and the hues defined by a locus of points that are each a common distance from a specified hue consist of hues that would each be perceived as differing from the specified hue to a common extent.
The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=A λ−5/(eB/T−1), where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody, and A and B are constants. Color coordinates that lie on or near the blackbody locus yield pleasing white light to a human observer. The 1931 CIE Diagram (FIG. 1) includes temperature listings along the blackbody locus (embodying a curved line emanating from the right corner). These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants which produce light that is on or near the blackbody locus can thus be described in terms of their color temperature. As used herein, the term “white light” refers to light that is perceived as white, is within 7 MacAdam ellipses of the black body locus on a 1931 CIE chromaticity diagram, and has a CCT ranging from 2000 K to 10,000 K.
Illumination with a CRI Ra of less than 50 is very poor and only used in applications where there is no alternative for economic issues. Lights with a CRI Ra between 70 and 80 have application for general illumination where the colors of objects are not important. For some general interior illumination, a CRI Ra>80 is acceptable. A light with color coordinates within 4 MacAdam step ellipses of the Planckian locus and a CRI Ra>85 is more suitable for general illumination purposes. CRI Ra>90 is preferable and provides greater color quality.
CRI only evaluates color rendering, or color fidelity, and ignores other aspects of color quality, such as chromatic discrimination and observer preferences. The Color Quality Scale (CQS) developed by National Institute of Standards and Technology (NIST) is designed to incorporate these other aspects of color appearance and address many of the shortcomings of the CRI, particularly with regard to solid-state lighting. The method for calculating the CQS is based on modifications to the method used for the CRI. CQS uses a set of 15 Munsell samples having much higher chroma than the CRI indices, and span the entire hue circle in approximately even spacing. CQS is calculated using the CIE 1976 L*a*b (CIELAB) uniform object color space, which is more uniform than the object color space (1964 W*U*V) utilized by CRI. CQS will only penalize a lamp's score for hue shifts, lightness shifts, and reductions in chroma. Lamps that increase object chroma relative to the reference illuminant are not penalized because these positive effects are generally preferred.
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode junction has been developed that can produce white light. A representative example of a white LED lamp includes a package of a blue LED chip (e.g., made of InGaN and/or GaN), coated with a phosphor (typically YAG:Ce or BOSE). Blue LEDs made from InGaN exhibit high efficiency (e.g., external quantum efficiency as high as 70%). In a blue LED/yellow phosphor lamp, a blue LED chip may produce an emission with a wavelength of about 450 nm, and the phosphor may produce yellow fluorescence with a peak wavelength of about 550 nm upon receipt of the blue emission. Part of the blue ray emitted from the blue LED chip passes through the phosphor, while another portion of the blue ray is absorbed by the phosphor, which becomes excited and emits a yellow ray. The viewer perceives an emitted mixture of blue and yellow light (sometimes termed ‘blue shifted yellow’ or ‘BSY’ light) as white light. Such light is typically perceived as cool white in color. A BSY device typically exhibits good efficacy but only medium CRI Ra (e.g., between 60 and 75), or very good CRI Ra and low efficacy. Cool white LEDs have a color temperature of approximately 5000K, which is generally not visually comfortable for general illumination, but may be desirable for the illumination of commercial goods or advertising and printed materials.
Various methods exist to enhance cool white light to increase its warmth. Acceptable color temperatures for indoor use are typically in a range of from 2700-3500K; however, warm white LED devices may be on the order of only half as efficient as cool white LED devices. To promote warm white colors, an orange phosphor or a combination of a red phosphor (e.g., CaAISiN3 (‘CASN’) based phosphor) and yellow phosphor (e.g., Ce:YAG or YAG:Ce) can be used in conjunction with a blue LED. Cool white emissions from a BSY element (including a blue emitter and yellow phosphor) may also be supplemented with a red LED (with such combination being referred to hereinafter as “BSY+R”), such as disclosed by U.S. Pat. No. 7,095,056 to Vitta, et al. and U.S. Pat. No. 7,213,940 to Negley et al., to provide warmer light. While such devices permit the correlative color temperature (CCT) to be changed, the CRI of such devices is reduced significantly at higher color temperatures.
Use of red and blue LEDs in the same device entails additional problems, since phosphide-based red LEDs exhibit substantially different changes in intensity and/or chromaticity than nitride-based blue LEDs with respect to changes in device operating temperature. That is, red LEDs include active regions typically formed of Group III phosphide (e.g., (Al,In,Ga)P) material, in contrast to blue LEDs, which include active regions typically are formed of Group III nitride materials (e.g., represented as (Al,In,Ga)N, including but not limited to InGaN). Group III phosphide materials typically exhibit substantially less temperature stability than Group III nitride materials. Due to their chemistry, red LEDs lose a significant portion (e.g., 40-50%) of their efficacy when operating at 85° C. versus operating at a cold condition (i.e., room temperature or less). When red and blue LEDs are in conductive thermal communication with one another (e.g., affixed to a common substrate or in thermal communication with a common heatsink), heat emanating from the blue LEDs will increase the temperature of the red LEDs. To maintain a relatively constant color point utilizing a device including a Group III-nitride-based blue LED (e.g., as part of a BSY emitter) and Group III-phosphide based red LED, current to the Group III-phosphide based red LED emitter must be altered as temperature increases because of the different temperature responses of the blue and red LED. Such current reduction results in reduction in total flux from the combination of emitters at a desired color point, limiting utility of such a device.
As an alternative to stimulating a yellow phosphor with a blue LED, another method for generating white emissions involves combined use of red, green, and blue (“RGB”) light emitting diodes in a single package. The combined spectral output of the red, green, and blue emitters may be perceived by a user as white light. Each “pure color” red, green, and blue diode typically has a full-width half-maximum (FWHM) wavelength range of from about 15 nm to about 30 nm. Due to the narrow FWHM values of these LEDs (particularly the green and red LEDs), aggregate emissions from the red, green, and blue LEDs exhibit very low color rendering in general illumination applications. Moreover, use of AlInGaP-based red LEDs entails the same limitations as noted above.
Another example of a known white LED lamp includes one or more ultraviolet (UV)-based LEDs combined with red, green, and blue phosphors. Such lamps typically provide reasonably high color rendering, but exhibit low efficacy due to substantial Stokes shift losses.
The therefore art continues to seek improved solid state lighting devices that address one or more limitations inherent to conventional devices. For example, it would be desirable to provide solid state lighting devices capable of providing white light in a wider variety of applications, with greater energy efficiency, with improved color rendering index over a range of correlative color temperatures, with improved efficacy, with improved color stability at high flux, and/or with longer duration of service.